DFT prediction of metallic conductivity and experimental investigation of air-induced degradation effects in quasi-one-dimensional antiferromagnet RbFeSe$_2$

A comprehensive study, combining density functional theory (DFT) calculations and experimental investigations, of the quasi-one-dimensional antiferromagnet RbFeSe$_2$ is carried out. The non-spin-polarized ab initio calculations show that its metallic conductivity is above the Néel temperature ${T_N = 248}$ K, with no gap in the electron density of states at the Fermi energy. The experimental four-probe conductivity measurements yet reveal an insulating behavior throughout the temperature range of $4$–$300$ K. Following these measurements, an X-ray diffraction analysis is conducted. Its results demonstrate a severe degradation of the sample after air exposure ($7$–$9$ min), with the reduction in selenium occupancy by more than $20\%$ below stoichiometric values and the formation of elemental selenium phase ($P3_221$ space group). The discrepancy between theoretical predictions and the obtained experimental results is attributed to the rapid air-induced oxidation leading to structural defects and electron localization. The results obtained highlight the critical importance of rigorous atmospheric control when studying iron chalcogenides, provide quantitative insights into the degradation mechanisms affecting electronic properties, and indicate that standard DFT approaches may overestimate metallicity in quasi-one-dimensional systems, particularly when structural defects are present.